Coaxial cavity tracking means and method



Feb. 6, 1962 BEISER 3,020,500

COAXIAL CAVITY TRACKING MEANS AND METHOD Filed May 20, 1960 HIS ATTORNEYUnit Sttes 3,020,500 COAXIAL CAVITY TRAQKING MEANS AND METHOD LeoBeiser, Flushing Manor, N.Y., assignor to Polarad ElectronicsCorporation, Long Island City, N.Y., a corporation of New York Filed May29, 196i Ser. No. 30,527 3 Claims. ((31. 33382) The present inventionrelates to coaxial resonators, and more particularly to new and noveltracking means for incrementally tuning the coaxial cavity to complywith a given error function.

Many microwave apparatus, in particular, field intensity meters andreceivers, include a plurality of separate tunable circuits, and itoften becomes important to be able to tune these various circuitssimultaneously while maintaining them in some predeterminedrelationship. For example, in a tuned high frequency type of receiver,various tuned circuits must be maintained with equal resonant frequency.In staggered-tuned circuits, such tuned circuits must be ideallymaintained with a constant difference between the resonant frequencies.Also, in the superheterodyne type of microwave receiver, a cm oscillatortuned circuit and a preselector tuned circuit must be tunable over theentire range of reception for the receiver, While simultaneouslymaintaining a relatively constant frequency difference between theresonant frequencies of the respectively tuned circuits. In themicrowave field these tuned circuits generally are of the cavityresonator type wherein the resonant frequency of a cavity is varied bychanging the effective electrical length of the transmission linedefined by the cavity dimensions. Thus a typical microwave receivermight include an external cavity klystron local oscillator, a trackingpreselector employing two resonant coaxial cavities coupled by means ofapertures to yield both the desired bandwidth and the desired rejectionof spurious signals, and other tracked circuit components such as dialmechanisms, a potentiometer to vary the klystron repeller voltage, etc.

- In many such applications as above described the tuned circuitconsists of a coaxial resonator wherein the tuning is accomplished bythe axial translation of the moveable center conductor with respect tothe outer conductor of the cavity. In such cases the variation inresonant frequency is a non-linear function of the displacement of thecontrol member, i.e., the moveable center conductor, and moreover thisfrequency vs. displacement relationship may differ for the various tunedcircuits to be simultaneously controlled. Accordingly, this gives riseto a problem known as tracking, by which is meant the maintenance of therespective tuned circuits in their proper resonant frequencyrelationship over the entire range of variation in tuning. Thus, in thecase of the cavity resonators employed in the klystron local oscillatorand the preselector of the typical microwave receiver abovementioned,where the respective resonators are tuned to different frequenciesrequiring difierent movement of their controlling members for the samefrequency change, the relationship of frequency vs. center conductordisplacement for each cavity is a complexly'variable one over'the rangeof frequencies involved. Therefore relatively elaborate means must beprovided for tuning the various resonant cavities and other trackedelements in the microwave apparatus in order to ensure that the desiredrelationshipover the tuning range exists between the respectivefrequencies, tuning dial indicators, repeller potentiometer, etc. Onesuch means for accomplishing this tracking embraces the use of aninitial or coarse cam for adjusting the displacement of the centerconductor in the resonant coaxial cavity to approximately tune thedevice. To

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obtain optimum performance it then becomes essential that the resonantfrequency of the cavity be adjusted by a fine correction cam which hasmany individual ad,- justment points and which superimposes anindividual corrective effect upon the tuning of the cavities by thecoarse cam in acting upon the displacement of the center conductor withrespect to the outer conductor. The inclusion of this last named cam,which accomplishes the fine tune ing of the coaxial cavity to complywith a given error function in order to maintain the proper trackingrelationship, is a technique in general use today in microwave equipmentwherein this tracking problem is present. Mechanical tracking of thistype suffers from many disadvantages among them the requirement that thedrive assembly be critically assembled to close dimensions andtolerances with resultant costly machining and alignment procedures.Also the complicated mechanical linkages involved in transferring thecompensation adjustment from the main drive to the incrementaladjustment of the tuning plunger in the coaxial cavity inherently leadsto residual backlash and dead spots. Further, such mechanical mechanismsare relatively expensive to fabricate because of the large number ofcomponents required. In many applications such as airborne equipment thegreat weight and size of the mechanical tracking adjustment. means is aserious factor. v

The present invention provides a novel electrica tracking compensatorfor a coaxial cavity, such as those used inpreselectors and otherstaggered-tuned circuits, to incrementally adjust the resonant frequencyof the cavity to comply with a given error function, i.e., to accomplishthe fine frequency tuning of a tracking tuned circuit. The coarse ormajor resonant tuning is first effected by a variation of the activelength of the center conductor inside the coaxial cavity by means of aconventional mechanical displacement linkage; The tracking compensationis then achieved by the use of a plurality of individually adjustablemembers staggered in space relationship along the outer conductor wallof the coaxial cavity in conjunction with a modification of the centerconductor or tuning plunger such that the electrical field configurationin the vicinity of the end point of the tuning plunger is influenced bythe depth of penetration of the particular adjustable member in registerwith the end point of the center conductor to thereby provide anincremental variation of the cavity field geometry and thus of itsresonant frequency. The invention provides an electrical trackingcompensator which has the advan-' tages of simplicity; economy; highresolution; and very significantly, a relative independence ofadjustment and effect of each discrete fine tuning member. The presentinvention is particularly useful in aligning and correcting the trackingof suchtuned circuits which are mass produced in quantity, whereinunavoidable variations in ,machining tolerances and electricalcharacteristics may be found, and which may thus be readily compensatedfor by the present invention.

It is therefore an objective of the invention to provide, a new andnovel means for tracking a coaxial cavity resonator.

' It is another object of the invention to provide a new and novel meansfor incrementally tuning a resonant, co-

axial cavity to comply with a given error function. It is yet anotherobjective of this invention to provide a tracking compensator which isparticularly suitable for use in multi-stage preselectors and otherstaggered micro wave tuned circuits where it is desirable to maintain apredetermined frequency relationship between the nant frequencies of therespective tuned circuits.

Still another objective of the present invention-is-toprovide such atracking compensator which is accurate;

economical to fabricate, simple in' construction, and of 3 much smallersize and weight than conventional mechanical tracking compeusators.

And a still further object of the present invention is to provide a newand novel tracking compensator for tunable coaxial cavities having aplurality of individually adjustable tuning corrections which arerelatively independent of each other.

Further other objects and advantages of the present invention willbecome apparent upon consideration of the following drawings whereinFIGURES l, 2, and 3, are representative plots of certain curves whichwill be useful in explaining the theory of operation of the invention;

FIGURE 4 is a front view, partially sectional, of a coaxial cavityresonator incorporating the present invention; and

FIGURE is a cross-sectional view as seen substantially along the planeof the line 55 in FIGURE 4.

The problem of the tracking of a coaxial cavity resonator so that it maybe incrementally tuned to comply with a given error function may beunderstood if reference is made to the following illustrative curves ofFIGURES 1, 2, and 3. FiGURE l is a representative plot of the resonantfrequency f vs. the active length l of the center conductor of a typicalcoaxial cavity resonator. As the tuning plunger or center conductor isaxially translated with reference to the outer conductor of the cavitythe resonant frequency thereof varies in the manner shown from arelatively high resonant frequency for a small active length of theplunger inside the cavity to a relatively low resonant frequency as thecenter conductor is extended to its fullest penetration inside thecoaxial cavity. It is to be noted that although the curve is non-linear;it is nevertheless smoothly continuous and represents an inversefrequency function, as may be seen.

FIGURE 2 is a representative curve of the error function A vs. frequencywhich arises in other tracked components of the microwave system due toreference miscalibration; i.e., the incremental error which arises inthe alignment of the tracked components of the system, such as themultiple tuned circuits having different tuning characteristics, thevariation of the potentiometer adjustment controlling the klystronrepeller voltage, the dial mechanism, etc. Thus FIGURE 2 isrepresentative of the tracking error which might result in a microwavesystem having a number of tracked components after the assembly ofcomponents and their gross alignment, by means of coupled mechanicaltranslating linkages (such as the cam and follower mechanismsabove-described), is made. The error function A vs. frequency f for themicrowave system may not follow any determinable relationship but mayvary randomly around the desired operating point Af=0; i.e., perfecttracking.

FIGURE 3 is a super-position of the error function M on the curve ofresonant frequency f vs. active length l of the center conductor insidethe coaxial cavity resonator. A combination of the two yields a plot ofthe desired variation in the center conductor travel vs. frequency forthe coaxial cavity resonator which would result in a perfect tracking ofthe system. Thus FIGURE 3 is suggestive of the prior art or mechanicalmethod of providing tracking for a tuned cavity-incrementa variation ofthe active length of the center conductor to achieve frequencycorrection by means of a fine correction cam having many individualadjustment points or other similar mechanical means for achievingindividual corrective effect upon the gross displacement by the main camof the tuning plunger or center conductor with respect to the outerconductor of the coaxial cavity resonator.

The present invention, however, provides an electrica tracking means,whereby the equivalent of corrective motion of the center conductor isaccomplished by an incremental variation of the cavity geometry, definedby the center conductor-to-wall spacing, insofar as the electrical fieldconfiguration in the vicinity of the end point of the tuning plunger isconcerned. Incremental tuning correction and hence tracking compensationare achieved by the resulting change in the capacity of the resonantcavity as the center conductor-to-wall spacing is varied. The errorcorrecting information is applied directly to the electrical fieldwithin the cavity such that a correction is accomplished at thefrequency of interest.

Referring again to the drawings, and particularly to FIGURES 4 and 5thereof, there is shown therein a coaxial cavity resonator, designatedgenerally as 10, having an outer conductor 11 with an end wall 12 and acenter conductor or tuning plunger 14. Apertures 13 are provided forinserting and extracting energy from the resonator in a conventionalmanner as is well understood by those cognizant of the art. Arrangedalong a line substantially parallel to the longitudinal axis of theouter conductor 11 and projecting therethrough, are a series ofnon-metallic members, designated as 16a 1611, made of some suitablematerial such as nylon, in spaced relationship. Each of these members16a 1612, which are shown, for example, in the form of a threaded setscrew having a charnfered tip portion 17 and a slotted head 18, arecapable of adjustment such that the depth of penetration of eachthreaded member into the cavity may be individually varied with relativefacility by rotation of a member 16 through means of its slotted head18. The tuning plunger or center conductor 14 has secured thereon springloaded metallic finger 20 which is affixed thereto by suitable meanssuch as rivets 22. Spring loaded finger 20 has a portion 24 which is incontact with the tip portion 17i of particular non-metallic member 161'in such a manner that said portion 24 is constantly urged thereagainstby reason of the retarding force exerted on the spring finger 20. Anaxial displacement of the center conductor 14 within the cavity by grosstuning means (not shown) accomplishes the major tuning of the resonantfrequency of the resonator 10. As such displacement takes place, it maybe seen that spring loaded finger 20 substantially terminating thecenter conductor 14 rides along the plurality of nonmetallic members 16a1611 by virtue of the spring engagement of the portion 24 and the tips17a 1711. Thus the spring loaded finger 20 is positioned in a directionnormal to the axis of the center conductor 14 as a direct function ofthe depth of penetration of the particular non-metallic member 16i inimmediate contact with the finger 20 at the point of contact 24. Themetallic spring loaded finger 20 afiixed near the termination of thecenter conductor 14- distorts the field configuration of theelectromagnetic energy mode propagated through the cavity resonator 10.

This distortion of the electrical field in the vicinity of the end ofthe tuning plunger may be described in terms of the end loaded capacityof the center conductor 14 to the other conductor wall 11. Any variationof the distance between the metallic finger 20 and the outer wall 11 ofthe cavity 10 would vary the electrical field configuration in theimmediate vicinity of the finger, and hence slightly affect (incomparison to the major tuning control) the resonant frequency f of thecavity, as will be readily understood by those conversant with the art.It may therefore be seen that a fine tuning for the resonator frequencycan be achieved by adjustment of the depth of penetration of the variousindividual non-metallic members 16a 16n.

Non-metallic members immediately adjacent to the particular member 16iin contact with the spring finger 20 at portion 24, such members beingdesignated for purposes of generality as 16i-1 and 16i+1, may have someeffect on the actuation of the incremental tuning at the point ofinterest depending upon the relative flatness of the portion 24 of thespring finger 20. This slight degree of carryover of adjustment frompoint to point may be necessary in order to avoid a scalloping effectbetween nonmetallic members due to discontinuous carryover from one tothe next as the tuning plunger 14 is axially translated.

Thus the invention provides a means of tracking compensation over aplurality of points throughout the frequency range in interest with onlya small degree of interdependence of adjustment from point to point. Byselective adjustment of each individual non-metallic member 161', incontact with the spring finger 20 at the particular resonant frequency tat which it is desired to have resonant cavity track the microwavesystem in which it is incorporated, it is possible to virtually reducethe error function Af to zero (corresponding to perfect tracking) at anumber of frequency points equal to the quantity 11, corresponding tothe number of nonmetallic members 16. This result is accomplished by aslight controllable distortion of the electrical field geometry in thevicinity of each one of the individual members 16a 16n, in the mannerabove-described, thereby to superimpose a fine tuning frequencycorrection on the major tuning of the coaxial cavity iii, as determinedby the active length l of the center conductor 14 within the cavitywalls.

It is thought that the foregoing description, if taken in conjunctionwith the annexed drawings, will be sufiicient to enable anyone practicedin the art to understand and construct a coaxial cavity trackingcompensator in ac cordance with the principles of this invention.Accordingly, it is desired that the invention be taken to embrace anyvariation and embellishment of the particular embodiment shown which iswithin the spirit thereof and that the scope of this invention belimited solely by the following appended claims.

What is claimed is:

1. A tracking compensator for incremental adjustment of the resonantfrequency of a tunable coaxial cavity comprising, a coaxial cavityhaving an outer conductor and a center conductor relatively moveablethereto along their common longitudinal axis, a metallic mass afiixednear the end point of said center conductor, and a plurality ofindividually adjustable members spaced in substantially axialprogression along said outer conductor and projecting therethrough suchthat at a plurality of operating positions of said center conductor withrespect to said outer conductor at least one of said individuallyadjustable members in the vicinity of the end point of said centerconductor cooperates in non-shorting fashion with said metallic mass,substantially independently of the remainder of saidindividually-adjustable members, to vary the effective electrical fieldconfiguration in said vicinity to provide thereby a relatively fineadjustment in 6 the frequency tuning of said cavity at said plurality ofoperating positions.

2. A tracking compensator for incremental adjustment of the resonantfrequency of a tunable coaxial cavity comprising, a coaxial cavityhaving an outer conductor and a center conductor relatively moveablethereto along their common longitudinal axis, a spring loaded metallicfinger member afiixed near the end point of said center conductor withinsaid cavity, a plurality of individually adjustable non-metallic membersspaced in substantially axial progression along said outer conductor andprojecting therethrough to a controllable depth of penetration into saidcavity, such that at least one of said individually adjustable membersin the vicinity of the end point of said center conductor cooperates,substantially' independently of the remainder of saidindividually-adjustable non-metallic members, with said spring loadedmember at a plurality of operating positions to vary the end loadedcapacity of said center conductor to said outer conductor wall toprovide thereby a relatively fine adjustment in the frequency tuning ofsaid cavity at said plurality of operating positions.

3. A tracking compensator for incremental adjustment of the resonantfrequency of a tunable coaxial cavity comprising, a coaxial cavityhaving an outer conductor and a center conductor relatively movablethereto along their common longitudinal axis, a conductivelaterallyextending member affixed to and movable with said centerconductor within said cavity and having a relatively short longitudinaldimension, a plurality of individually adjustable members axially spacedin progression along said outer conductor and projecting therethrough toa controllable depth of penetration into said cavity to cause the endloaded capacity of said center conductor to said outer conductor wall tobe substantially individually controlled by each adjustable member assaid laterally extending member assumes respective ones of a pluralityof operating positions corresponding to the axial position of arespective adjustable member, whereby a fine adjustment in the frequencytuning of said cavity is provided at said plurality of operatingpositions.

References Cited in the file of this patent UNITED STATES PATENTS2,566,759 Clark Sept. 4, 1951 2,600,278 Smullin June 10, 1952 2,639,406Crawford May 19, 1953 2,851,666 Kach Sept. 9, 1958

